Hydrophilic Metal Nanoparticles Functionalized by 2‑Diethylaminoethanethiol: A Close Look at the Metal−Ligand Interaction and Interface Chemical Structure Iole Venditti, † Giovanna Testa, † Fabio Sciubba, † Laura Carlini, ‡ Francesco Porcaro, ‡ Carlo Meneghini, ‡ Settimio Mobilio, ‡ Chiara Battocchio,* ,‡ and Ilaria Fratoddi* ,† † Department of Chemistry, Sapienza University of Rome, P.le A. Moro 5, 00185 Rome, Italy ‡ Department of Science, Roma Tre University of Rome Via della Vasca Navale 79, 00146 Rome, Italy *S Supporting Information ABSTRACT: Hydrophilic gold and silver nanoparticles stabilized with 2-diethylaminoethanethiol hydrochloride (DEA) have been prepared and characterized. AuNPs-DEA and AgNPs-DEA with mean diameter below 10 nm have been characterized by means of dynamic light scattering and field- emission scanning electron microscopy techniques. Nuclear magnetic resonance (NMR) studies allowed to assess transla- tional mobility, aggregation equilibrium in function of pH variations and presence of chemisorbed and physisorbed thiol molecules; in particular ethyl groups on DEA ligands are free to rotate, suggesting a rather loose packing of the thiols on the nanoparticle surface. NMR results were compared with X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure, and X-ray absorption spectroscopy. The complementary information acquired allowed to obtain information on the interaction at the interface between the organic thiol ligand and metal nanoparticles (NPs) at atomic level as well as on the molecular structure. The influence of the thickness of the functionalizing layer on the stability of NPs has been studied and opened new insight on the local structure surrounding the NPs. ■ INTRODUCTION Functionalized metal nanoparticles (MNPs) with average size ranging from units to tens of nanometers are considered emerging materials for advanced applications in catalysis, 1,2 optoelectronics, 3 sensors, 4 and biomedicine 5,6 from drug delivery 7 to diagnostics. 8 In-depth investigations focused on their chemico−physical characteristics, highlighting the role of size and shape dependence of surface plasmon resonance (SPR) and electronic properties on the nanoscale. 9−11 Among others, gold and silver nanoparticles (AuNPs and AgNPs) can be stabilized by a variety of ligands with chemical ending functionalities purposely chosen for the specific application. 12,13 Citrate-stabilized AuNPs can be considered among the most popular ones, and remarkable studies evidenced the role of citrate concentration in the size and dispersion of AuNPs; that is, a high concentration of citrate usually gives rise to small size NPs, whereas a low concentration leads to larger NPs and aggregation phenomena. 14 The use of thiols has been extensively exploited from the pioneering studies on alkanethiols in the Shiffrin−Brust two- phase route, particularly suited to fine-tune nanoparticles’ size and shape, 15 and the overall mechanism including all of the steps of the AuNPs synthesis has been demonstrated by Raman and nuclear magnetic resonance spectroscopies. 16 For example, 3-mercaptobenzoic acid (3-MBA) has been extensively studied, gaining insights into the formation mechanism of Au nanoclusters. 17 The availability of functional thiols opened new perspectives for the achievement of nanoparticles soluble in different environments by using hydrophilic 18,19 or hydro- phobic thiols; 20 bifunctional thiols have been used for the achievement of interconnected networks. 21,22 The Shiffrin− Brust method was extended to single-phase systems by selecting thiols soluble in the same solvent as HAuCl 4 and avoiding the introduction of phase-transfer agents such as tetraoctylammonium bromide (TOAB). In particular, water- soluble MNPs are of great biomedical interest 23−25 and have been obtained by direct synthesis or ligand-exchange reactions. For example, water-soluble AuNPs have been obtained with low size dispersion 26 by direct reduction of aqueous solutions of HAuCl 4 , and ligand exchange approach has been applied to obtain small core-size nanoparticles, 27 although the extent of ligand exchange depends on the nature of the incoming ligand. Biocompatibility and toxicity tests have been carried out, giving evidence of the mechanism of their cellular uptake and biodistribution, which mainly depends on the characteristics of the MNPs surface. 28,29 AuNPs can be used to selectively Received: February 13, 2017 Revised: March 23, 2017 Published: March 23, 2017 Article pubs.acs.org/JPCC © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.7b01424 J. Phys. Chem. C XXXX, XXX, XXX−XXX